Use of liposomes as a drug delivery system seems promising. However current formulations of liposomes are taken up primarily by phagocytic cells of the reticuloendothelial system. This project is designed to investigate and control interactions between liposomes and reticuloendothelial cells both in vitro and in vivo. Thus studies will be made of liposome - macrophage interactions in vitro and of the clearance and distribution of liposomes in rats. Effects of changing liposome physical properties and of changing serum factors will be studied.